In relation to some diseases patients must inject a medicament on a regular basis such as once weekly, once daily or even a plurality of times each day. In order to help patients overcome fear of needles, fully automatic injection devices have been developed that makes the use of an injection device as simple as possible. Such devices are typically designed such that a user shall position the injection device onto the injection site and activate the device. Such activation causes the device to insert a needle into the skin, eject a dose of the medicament and subsequently move the needle into a shielded position.
An example of such a device is shown in EP516473A1 which employs a powerful spring that, when released, thrusts forward the piston of a syringe to thereby project the needle of the syringe in the flesh of the patient and subsequently thrust forward the piston inside the syringe to expel the medicament. At the end of this phase the spring is automatically decoupled from the piston, leaving the syringe free to be acted upon by a relatively weak return spring, which urges the syringe to a retracted position to thereby shield the needle.
As identified in WO 03/097133, an injection device of the above type, have a problem with dosage delivery due to length tolerances of the syringe and due to the exact trigger point for triggering of the needle retraction being very much dependent on the length of the syringe. WO 03/097133 attempts to alleviate this problem by introducing a two part piston drive where the two parts of the piston drive are connected by means of a damper mechanism. This ensures that the piston is moved all the way to the needle end of the syringe before triggering the needle retraction sequence. However, due to the damping system is initiated upon the initial actuation of the device, the point in time where the needle retraction actually is initiated is not well defined relative to the point in time where the piston impacts the internal end walls of the syringe. This may still lead to inaccurate delivery and/or prolonged waiting time for an administration to be completed. In related auto-injectors such as disclosed in WO 03/092771 and WO 2008/029280 similar problems apply.
In other injection devices, such as the ones disclosed in WO 2006/062997, the piston stroke of a cartridge is mainly controlled by using a rear part of the cartridge to define both the start point for the piston and the end of stroke point where the expelling movement is interrupted. The disclosed injectors provide auto penetration and auto expelling features but require manual withdrawal of the needle from the skin subsequent to an injection.
When using injectors where the patient is responsible for manually removing the needle from the skin after the end of an injection, the patient is generally requested to leave the needle inserted for 6 to 10 seconds, or even longer after the end of stroke condition, to ensure that the complete desired amount of injectable fluid is actually injected. During this time, the system relaxes and a small amount of drug will be forced out through the needle. This effect is partly attributable to the fact that during the dosing procedure, the piston deforms, which results in the actual travelled distance of the front face of the piston being different than the travelled distance of the piston driver immediately after the piston driver has travelled the predetermined distance. After the piston driver has stopped its movement the piston will return to its original shape, thus ejecting the remainder of the medicament. This phenomenon is causing some inconvenience for the user. He needs to keep the needle in the skin for a relatively long time in order to fully receive the intended dose.
In a further reference, WO 2008/020023, the problem of drooling from the needle after an injection procedure is addressed by proposing to release the pressure on a piston rod of the device shortly after injection. In addition, this reference proposes to include a valve at the delivery opening of the device for preventing drooling at the end of a delivered dose. Both solutions result in unnecessary complex devices.
Having regard to the above-identified prior art devices, it is an object of the present invention to provide an injection device which minimizes the time that the needle need to be inserted into the skin, yet obtaining a large degree of dosing accuracy and automation.
For injection devices, in particular for single shot injectors, different references in the art proposes to incorporate injection needles of the type having a penetrable sterility sheath which encircles the needle cannula at least along a part of the extension of the cannula. The sterility seal of such injection needles enables penetration by the pointed tip of a needle cannula, when the sterility sheath is moved against the pointed tip. Such injection needles typically offer a minimum of required handling steps during the act of administering a dose. However, this kind of injection needles typically require that each injection needle, during manufacture, is handled separately from other injection needles in order not to accidentally penetrate the sterility sheath. It is a further object of the present invention to provide a cost-effective sterilization and handling process for such injection needles.
Yet additional further objects of the invention are to provide measures for obtaining devices having superior performance and, at the same time, enabling manufacture at a reduced cost.